Google has done a good job at keeping data-stealing Trojan apps out of Google Play, but attackers still find ways to monetize rogue apps through the store.

Avast Software researchers recently found three apps on Google Play with hidden adware functionality that was designed to activate days after the apps were installed. The rogue applications -- a game called Durak, an IQ test and a history app -- have been downloaded millions of times.

When users first install Durak, it looks and acts like a normal gaming app, Avast researcher Filip Chytry said in a blog post Tuesday. "This impression remains until you reboot your device and wait for a couple of days. After a week, you might start to feel there is something wrong with your device."

Specifically, every time users unlock their phones, the app displays persistent ads claiming the device and its data are at risk.

Users are asked to act, but if they do, they run into real trouble, according to the researcher. For example, they may get redirected to questionable app stores and to apps that surreptitiously attempt to send premium text messages on behalf of the users. People may also encounter apps that collect too much of their information without offering much value.

If this sounds familiar, it's because the scheme is similar to the highly effective scareware scams that have plagued PC users for years by spooking them into installing rogue antivirus programs or system optimization tools using fake warnings.

Delaying the warning messages for several days is a clever technique by the rogue developers because users will have a hard time determining which app is responsible for the alerts, and that's assuming they even suspect that the messages are triggered by an app.

Also, apps uploaded to Google Play are scanned inside an Android emulator called Bouncer to observe their post-installation behavior. By delaying the malicious activity, the app authors likely hope to bypass this behavior-based analysis.

"I believe that most people will trust that there is a problem that can be solved with one of the apps' advertised 'solutions' and will follow the recommended steps, which may lead to an investment into unwanted apps from untrusted sources," Chytry said.

In some cases, the rogue ads directed users to legitimate security apps that were also hosted on Google Play, probably in an attempt to earn money through referral schemes.

Google removed the three offending applications identified by Avast from Google Play. However, the incident shows that although Trojans account for most Android malware, other types of threats also lurk on the official app store.

Google confirmed the apps have been suspended but offered no comment about this type of threat nor about how attackers are able to bypass Google Play defenses.